Alternating current induction motors are widely used to drive many types of equipment. An alternating current induction motor of the usual type tends to be inherently a fixed speed motor with the speed related to the line frequency.
There are many occasions in which it is desirable to vary the speed of the driven machinery or equipment. In the case of belt-driven machinery, a belt sheave can often be changed to accomodate the changed speed requirement. In the case of gear or chain-driven equipment, it is often possible to change gears or sprockets to arrive at the desired speed of the driven equipment. Many times, however, the equipment must be direct driven. Even in a case of drive through belts, gears, or chains it may not be possible to change sheaves or gears, or sprockets as a necessary size may not be available commercially or the necessary size may not physically fit into the existing space. In other occasions, the mechanical solution may be cost prohibitive, due to the cost of labor or materials, or a combination of both.
One possible option is to replace the alternating current induction motor with one designed to run at a different speed, or one designed to run at various speed ranges. Sometimes a variable speed direct current motor can be substituted. Or, to make use of the A/C motor, there are commercially available solid state frequency controllers that provide a continuously variable frequency output from zero to line frequency or beyond. Although any of these solutions would work, any or all frequently are financially prohibitive for a given installation.
One common installation in which slowing the speed of an A/C induction motor is desirable, but wherein economic conditions generally prohibit the foregoing solutions is found in an oil field. A walking beam-type pumping unit is commonly operated by a three phase 1800 r.p.m (or other speed) A/C induction motor and is commonly used to withdraw oil from a well. As the oil field becomes depleted, the casing of a given pump cannot fill up from the oil field as fast as the pump tries to pump the oil from the ground. This produces a situation known as pumpdown, and unless the pump is shut down for a period of time so that the formation can "catch up", severe wear and tear will ensue on the pump parts, and energy will be consumed unnecessarily moving a pump up and down a hole without producing any oil. This situation is often accommodated by cycling the pump on and off with timers or other control devices. This is undesirable as starting and stopping of the motor and the pump causes shock loads that produce far more wear and tear on the machinery than does continuous running. It is far more desirable and more efficient to solve the pumpdown problem by slowing the pump to match the oil formation production rate, so that the oil is pumped from the well at essentially the same rate as it flows into the casing from the formation. This can sometimes be done by changing the motor sheave, but sometimes the sheave in use is the smallest that is available, and the pump cannot be slowed down simply by a change of sheaves. The economics of a commercially depleted oil field may not justify purchasing and installing a new slower speed motor, or one of the commercially available variable frequency drive systems now on the market.
Another example of the desirability of speed control is found in the heating and air conditioning industry. An air conditioning compressor must be designed to run essentially full time for the maximum load that it will encounter. However, on days when temperatures are below the maximum for which the installation is designed, overcooling may occur unless the air conditioning compressor and ventilating fans are cycled on and off. This cycling is effected with thermostatic controls, and the life of the equipment is reduced, and energy costs are increased through this inefficient mode of operation. Futhermore, the resultant cycling of temperature can be uncomfortable and is undesirable. A preferred mode of operation would be to slow the compressor and/or fan so that the BTU output of the air conditioning system would more closely match the heat loss characteristics of the building in which it is installed. With technology now available, it is possible for these motors to be run at substantially any speed from zero to full rated r.p.m. The problem lies in that the equipment available is high priced and economically unfeasible, and the motors continue to be cycled on and off in an inefficient manner.